Supply voltage reference circuit

Abstract

A supply reference voltage circuit is coupled to an output node, a supply voltage node and a supply reference voltage node and is operable to connect the output node to the supply reference voltage node and prevent current flow through an output device coupled to the output node in response to sensing a low voltage level at the supply voltage node and a non-zero voltage at the output node. The circuit is further operable to connect the supply reference voltage node to the supply voltage node in response to the voltage at the output node being a threshold voltage above the voltage at the supply voltage node. The circuit is further operable to bypass a blocking diode in response to sensing a high voltage level at the supply voltage node.

What is claimed is:  
     
       1. A method of providing a supply reference voltage, comprising: 
       turning off an output transistor in response to sensing a non-zero voltage level at an output node and a zero supply voltage level;  
       connecting the supply reference voltage to the voltage at the output node coupled to the output transistor in response to sensing the voltage at the output node being a threshold voltage above the supply voltage level; and  
       providing a path from a supply voltage to the supply reference voltage and thereby bypassing a current blocking diode coupled between the supply voltage and the supply reference voltage.  
     
     
       2. The method, as set forth in  claim 1 , wherein turning off the output transistor comprises supplying the non-zero voltage level at the output node to the supply reference voltage, the output transistor being in an OFF state when the supply reference voltage coupled to its gate is high. 
     
     
       3. The method, as set forth in  claim 1 , wherein connecting the supply reference voltage to the voltage at an output node comprises: 
       turning on a first p-channel transistor coupled between a first control node and the output node in response to sensing the supply voltage coupled to its gate being zero; and  
       turning on a second p-channel transistor coupled between the first control node and the supply reference voltage in response to sending the supply voltage coupled to its gate being zero.  
     
     
       4. The method, as set forth in  claim 1 , wherein providing a path from a supply voltage to the supply reference voltage comprises turning on a p-channel transistor coupled between the supply voltage and the supply reference voltage in response to sensing a high at the supply voltage. 
     
     
       5. The method, as set forth in  claim 1 , comprising: 
       controlling a first p-channel transistor coupled between a first control node and the output node with the voltage level of the supply voltage node coupled to its gate;  
       controlling a second p-channel transistor coupled between the first control node and the supply reference voltage node with the voltage level of the supply voltage coupled to its gate;  
       controlling a third p-channel transistor coupled between a second control node and the output node with the voltage level of the supply voltage node coupled to its gate;  
       controlling a first n-channel transistor operable to pass an inverse of the supply voltage with the voltage level at its gate coupled an inverse of the ouput voltage;  
       controlling a bypass p-channel transistor coupled between the supply voltage node and the supply reference voltage node with the voltage level at the first control node coupled to its gate; and  
       controlling a second n-channel transistor coupled between the second control node and the output node with the voltage level of the supply voltage node coupled to its gate.